Add ability to Ignore arguments

[masonium]

This PR adds an Ignore parameter to the memoize macro, allowing the user to mark one or more function arguments to not participate in the memoization. As a result, any Ignored argument does not need to implement Clone or Hash, and changes to those arguments do not trigger re-invoking the original function.

[dermesser]

Looks good - but this feature isn't gated on the full feature, is it? that's what makes the CI fail currently.

[masonium]

Thanks, yeah I forgot to remove the 'full' feature gate from the example, and it doesn't require it.

[masonium]

Sorry for the churn here. I found the issue with my most recent branch: I was checking for the first argument using sig.inputs[0] instead of sig.inputs.first(), which obviously fails for functions with no arguments.

I also ran the tests locally matching the matrix used in .github/workflows/test.yml, so the CI should go through.

[masonium]

Let me know if you need anything else from the PR. Happy holidays!